"Falcarius is kind of half-raptor and half-herbivore. This transition is triggered by a shift in diet."

He says it appeared at around the time that tasty, nutritious, flowering plants appeared on Earth.

"We know that the first dinosaur was a small-bodied, lightly built, fleet-footed predator," Sampson adds. All other dinosaurs evolved from it.

"However, as with many radiations of major groups of animals, it happened so quickly that we really don't have much in the way of fossil documentation."

Falcarius provides part of the picture, he says.

The adult Falcarius would have walked on two legs and was about 4 metres long and 1.4 metres tall.

It had strong forearms, sharp, curved, 10 centimetre claws and a long neck. And it was probably covered with shaggy, hair-like "proto-feathers".

Feathered fiend?

Falcarius is the earliest North American example of a therizinosaur, a group that includes feathered dinosaurs found in southeast China and maniraptorans, including the velociraptor.

"[It] is the most primitive known therizinosaur, demonstrating unequivocally that this large-bodied bizarre herbivorous group of dinosaurs came from velociraptor-like ancestors," says Lindsay Zanno, a graduate student in geology and geophysics who worked on the study.

She describes it as "the ultimate in bizarre ... a cross between an ostrich, a gorilla and Edward Scissorhands", a film character that had scissors instead of hands.

"Falcarius shows the beginning of features we associate with plant-eating dinosaurs, including a reduction in size of meat-cutting teeth to leaf-shredding teeth, the expansion of the gut to a size needed to ferment plants, and the early stages of changing the legs so they could carry a bulky body instead of running fast after prey," says Dr James Kirkland, Utah state paleontologist at the Utah Geological Survey.

The fossils were excavated from ancient gravel at the Crystal Geyser Quarry, which produces cold water and carbon dioxide gas.

"A bunch of these animals were killed more than once," Kirkland says, adding that "hundreds, perhaps even thousands" of individual fossils were found.

Kirkland believes the animals perhaps lived in flocks or herds, were attracted to plants around the spring and occasionally poisoned en masse by gas or contaminated water.

"Mass mortalities are known in a number of dinosaur groups, both meat-eating and plant-eating dinosaurs," he says.

In the present-day area periodic outbreaks of botulism poisoning have been documented, he says.

Other possible explanations for the large number of fossils found at the site include drought, volcanic activity and fire, the scientists say.